Technical Field
This patent application relates generally to radio frequency and microwave frequency filters and more particularly to a filter using variable impedance transmission line sections to synthesize discrete inductive and capacitive elements.
Background Information
Consumer electronic devices such as smart phones, tablets, laptop computers, wireless headphones, video displays and the like are now typically expected to operate using many different radio frequency (RF) and/or microwave frequency communication protocols over a wide range of frequencies. There is increasing pressure for such products to be as small and inexpensive as possible.
These and other electronic products thus require some sort of circuit and/or device to meet the resulting stringent requirements for selecting and controlling frequency. A circuit or component capable of performing frequency selection is called a filter. Filters may be classified based upon how they modify the frequency spectrum of the input signal. Low pass, high pass, band pass, and stop band are some of the more common classes of filters.
Filters may be implemented as a passive design using discrete lumped reactive elements such as inductors and capacitors. The discrete elements can be laid out in a number of different topologies including L-section, T-section, pi-section, ladder-networks, and so forth. Discrete element filters may also use active components such as amplifiers. Filters may also be implemented with specialized structure(s) that take advantage of the electromagnetic effects of various materials and/or physical arrangement of components. Other considerations in filter design include the order, namely the number of poles and zeros, which further determine the characteristics of the frequency response.
Tunable lumped element filters can be implemented using adjustable reactive components such as variable inductors and/or variable capacitors. Tunable filter structures are also known that use mechanical adjustment or electromagnetic effects such as Bragg reflection to effect a change in frequency.